This invention relates to improved methylene acceptors having a low molecular weight, a low free resorcinol content, and a low softening point. The derivatives of the present invention provide low fuming and low viscosity in the uncured rubber compound, good adhesion for steel cord, high dynamic storage modulus in the cured rubber product and ease of processing. In addition, they are non-hazardous to produce.
Resorcinol (molecular weight 110; melting point 110xc2x0 C.) has long been used in the rubber industry as a reinforcing and bonding agent to promote adhesion between steel wire and rubber compounds. Resorcinol is referred to as a methylene acceptor because it reacts with a methylene donor, a compound that generates methylene or methylol groups capable of crosslinking with the methylene acceptor. The methylene donor and methylene acceptor crosslink during rubber processing to form in situ a resin that promotes adhesion of the rubber to reinforcing materials such as metal wire, glass, organic filaments, fibers, cords and fabrics.
Because it is a small molecule, resorcinol can become uniformly distributed among the rubber molecules during compounding with rubber, resulting in an uncured rubber mixture that exhibits very low viscosity. This is a desirable property, as it results in greater ease of processing and extrusion of the uncured rubber. However, even though resorcinol provides excellent physical, mechanical and steel adhesion properties in the uncured and cured rubber products, fuming associated with resorcinol at rubber processing temperatures causes problems, particularly with loss of product. For example, slight variations in the processing temperature can result in variations in the amount of resorcinol in, and properties of, the end product.
To overcome the resorcinol fuming problem, derivatives of resorcinol have been used as methylene acceptors in rubber processing. Various resorcinol-formaldehyde resins which can act as suitable methylene acceptors have also been developed, reducing the free resorcinol content and the associated resorcinol fuming. See, for example, the following related patents:
U.S. Pat. No. 4,476,262 discloses a water dilutable resinous product prepared by reacting together a resin, an aldehyde and a sulfurous or organic acid.
U.S. Pat. No. 4,605,696 discloses use of monoesters of resorcinol, including resorcinol monobenzoate and resorcinol monorosinate, in rubber compositions.
U.S. Pat. No. 4,731,430 discloses phenol resins modified with compounds possessing amide and/or imide groups that are useful as crosslinking agents and in the preparation of adhesives and rubber assistants.
U.S. Pat. No. 4,889,891 discloses alkyl substituted resorcinolic novolak resins as suitable methylene acceptors for vulcanizable rubber compositions.
U.S. Pat. No. 4,892,908 discloses the use of keto derivatives of resorcinol, such as benzoyl resorcinol, as methylene acceptors in vulcanizable rubber compositions.
U.S. Pat. No. 5,021,522 discloses aralkyl substituted resorcinolic novolak resins including a styrene substituted resorcinol-formaldehyde resin.
U.S. Pat. No. 5,030,692 discloses alkylphenol modified resorcinolic novolak resins.
U.S. Pat. No. 5,244,725 discloses a vulcanizable rubber composition that includes a rubber compound, a methylene donor and a methylene acceptor.
One problem with resorcinol-formaldehyde resins is diminished rubber compounding properties because the formaldehyde molecule contains two sites of reactivity. Formaldehyde will crosslink two resorcinol molecules, resulting in a higher molecular weight resin. Resorcinol resins of higher molecular weight have less reactive sites with which to crosslink the methylene donor, do not disperse as well in the rubber, and increase the viscosity of the uncured rubber.
Other methods of producing resorcinolic resins, such as by adding styrene to resorcinol before reaction with formaldehyde, or by using an epoxy reaction followed by styrenation, have been attempted. The methylene acceptors produced by these methods are of relatively high molecular weight, and have the problems mentioned above. None have achieved the desired product or processing properties in combination with low molecular weight, low fuming and low free resorcinol content.
The present invention solves the above need by providing a method of making a low molecular weight methylene acceptor having physical and mechanical properties similar to resorcinol, but without the fuming problems, for use in rubber processing. The methylene acceptor is prepared by reacting a polyhydric phenol with an aromatic olefinic compound in the presence of an acid catalyst to produce a first reaction product, and then further reacting the first reaction product with a monofunctional compound which is an N-methylol lactam derivative, such as propiolactam, butyrolactam, valerolactam, caprolactam, and heptalactam. The resulting methylene acceptor has a low molecular weight, low softening point, and a low free resorcinol content. Additional advantages include the formation of water as a by-product, as compared with by-products such as alcohol or other organic compounds produced in prior art methods, and the absence of a need for a co-solvent such as xylene or toluene. The method of the present invention can use existing plant conditions, without the need for re-engineering. Derivatives produced by the above-described method are also provided in the present invention.
The present invention further provides a rubber composition comprising: (a) a rubber compound; (b) a methylene donor; and (c) a methylene acceptor; the methylene acceptor is prepared by reacting a polyhydric phenol with an aromatic olefinic compound in the presence of an acid catalyst to produce a first reaction product, and then further reacting the first reaction product with an N-methylol lactam derivative, such as propiolactam, butyrolactam, valerolactam, caprolactam, and heptalactam, to produce a second reaction product which is the methylene acceptor. Rubber compositions further comprising a reinforcing material are also provided by the present invention.
It is an object of the present invention, therefore, to provide a low molecular weight methylene acceptor.
It is an additional object of the present invention to provide styrenated (polyhydroxy N-benzyl) lactam derivatives.
It is an additional object of the present invention to provide a methylene acceptor with low free resorcinol content to reduce fuming during rubber processing.
It is a further object of the present invention to provide a methylene acceptor which is a low melting solid, to improve ease of processing and handling.
It is another object of the present invention to provide a methylene acceptor with bulkier substituents to provide high dynamic storage modulus in the cured rubber compound.
It is an additional object of the present invention to provide a methylene acceptor made by a simple and nonhazardous process.
These and other objects of the invention will be more fully understood from the following description of the invention and the claims appended hereto.
The present invention provides a method of making a low molecular weight methylene acceptor comprising reacting about 1.0 mole of a polyhydric phenol with about 0.1-1.2 moles of an aromatic olefinic compound in the presence of an acid catalyst to produce a first reaction product, and further reacting this first reaction product with about 0.1-1.2 moles of an N-methylol lactam derivative, such as propiolactam, butyrolactam, valerolactam, caprolactam, and heptalactam, to produce a second reaction product which is the methylene acceptor of the present invention. Thus, the molar ratio of polyhydric phenol:aromatic olefin:N-methylol lactam derivative is about 1:0.1-1.2:0.1-1.2.
As used herein, the term xe2x80x9clow molecular weight methylene acceptorxe2x80x9d refers to the second reaction product, made as described above, which includes derivatives of the polyhydric phenol which are lower in molecular weight than the commonly used resorcinol-formaldehyde resins. The second reaction product will comprise a blend of compounds, including a styrenated polyhydric phenol - lactam derivative, a non-styrenated polyhydric phenol - lactam derivative, a styrenated polyhydric phenol, and unreacted polyhydric phenol. The mixture may further contain various di-substitution products, particularly when higher molar ratios are used. While the term xe2x80x9cstyrenatedxe2x80x9d is used to describe the reaction of styrene with the polyhydric phenol, it will be understood that other aromatic olefins can be used, as more fully described below. Within the above molar ranges, higher amounts of the aromatic olefin and the N-methylol lactam will result in the styrenated polyhydric phenol-N-methylol lactam derivative as the predominant structure, as determined by spectroscopic analysis. Also present in the second reaction product are various structures resulting from the decomposition of the N-methylol group. Amounts of reaction and decomposition products will vary depending on the exact molar ratios used and the reaction conditions.
The polyhydric phenols of the present invention include, but are not limited to, resorcinol, catechol, dihydroxybiphenyl, trihydroxybiphenyl, hydroquinone, alkylidenebisphenols or thio-bisphenols. The alkylidene group of the alkylidenebisphenols can have from about 1 to 12 carbon atoms. The alkylidenebisphenols include, but are not limited to, 4,4xe2x80x2-methylenediphenol (bisphenol F), and 4,4xe2x80x2-isopropylidenediphenol (bisphenol A). Also within the scope of the present invention are polyhydric phenols that are substituted by at least one of the groups including an alkyl group having from about 1 to 12 carbon atoms, an aralkyl group having from about 6 to 12 carbon atoms, an alkanoyl group having from about 2 to 18 carbon atoms, an aroyl group having from about 7 to 11 carbon atoms or a halogen selected from the group consisting of chlorine and bromine. Preferred are dihydric phenols; most preferred is resorcinol.
The aromatic olefinic compounds as used in the present invention 
include any aromatic olefinic compounds of the general formula (1)
wherein R is selected from the group consisting of H and CH3 and R1 is independently selected from the group consisting of H, OH, an alkyl group having from 1 to 6 carbons, a halogen and xe2x80x94CHxe2x95x90CH2. Preferably, the aromatic olefinic compound is an aromatic vinyl compound including alpha-methylstyrene, p-methylstyrene, alpha-chlorostyrene, styrene, divinylbenzene and vinyl napthalenes. Most preferred is styrene.
N-methylol caprolactam and N-methylol butyrolactam are the preferred N-methylol lactams for use in creating the methylene acceptor of the present invention. The N-methylol caprolactam and the N-methylol butyrolactam are represented by the following formulas (2) and (3), respectively: 
Also included within the scope of the present invention are the N-methylol lactams represented by the following chemical formulas: N-methylol propiolactam (formula 4); N-methylol valerolactam (formula 5); and N-methylol heptalactam (formula 6), which can also be used to create the low molecular weight methylene acceptor: 
Unlike formaldehyde, these monofunctional molecules will react with the polyhydric phenol to produce a simple substitution product. Crosslinking of the aromatics is avoided, leading to a blend of low molecular weight derivatives having low free polyhydric phenol, a low softening point and other desired properties for rubber processing.
As described above, the low molecular weight methylene acceptor will comprise the blend of compounds listed above. In a preferred embodiment, resorcinol is used as the polyhydric phenol, styrene is the aromatic olefin and an N-methylol lactam derivative is used (as described above, N-methylol caprolactam and N-methylol butyrolactam are the preferred). The predominant reaction product is a styrenated resorcinol lactam derivative and is represented by the following chemical formula (7): 
wherein R2 is an N-methylene lactam group and is represented by the formula: 
and n=2-6.
The molecule may also be di-substituted; as described above, the reaction mixture may contain a variety of other compounds in varying amounts.
The reaction of the polyhydric phenol with the aromatic olefinic compound according to the methods of the present invention is preferably carried out in the presence of an acid catalyst. Suitable catalysts include, but are not limited to, H2SO4, H3PO4, aromatic and aliphatic sulfonic acids, and the like. The preferred catalyst is p-toluenesulfonic acid (PTSA). Typically, the reaction should take place at a temperature of between about 80xc2x0 and 180xc2x0 C., preferably between about 125xc2x0 and 135xc2x0 C. The reaction time is between about 2 to 5 hours, preferably about 2-3 hours. The second phase of the reaction, the reaction of the lactam derivative with the first reaction product, occurs at a temperature of between about 80xc2x0 and 120xc2x0 C. The reaction time is between about xc2xd to 3 hours, preferably between about xc2xd to 1xc2xd hours.
In a preferred embodiment of the method of the present invention, resorcinol is reacted with styrene in the presence of a PTSA catalyst. The reaction is performed at a temperature of between about 125xc2x0-135xc2x0 C. for a period of between about 2xc2xd to 3 hours. N-methylol caprolactam is then added slowly to the reaction mixture while maintaining the temperature at between about 95xc2x0 to 120xc2x0, for a period of between about xc2xd to 1xc2xd hours.
The present invention is further directed to a vulcanizable rubber composition having improvements in physical and mechanical properties such as dynamic stiffness, hardness, scorch safety and cure time. The vulcanizable rubber composition of the present invention comprises: (a) a rubber compound selected from natural rubber, synthetic rubber or combinations thereof; (b) a methylene donor; and (c) a methylene acceptor. The rubber composition will also contain other additives. The methylene acceptor is prepared by reacting a polyhydric phenol with an aromatic olefin in the presence of an acid catalyst to produce a first reaction product, followed by further reacting the first reaction product with an N-methylol lactam derivative, such as propiolactam, butyrolactam, valerolactam, caprolactam, and heptalactam, to produce a second reaction product. Thus, the second reaction product is the low molecular weight methylene acceptor prepared by the above-described methods.
Any suitable methylene donor can be used. Preferred are 2-nitro-2-methyl-1-propanol, hexamethylenetetramine (HMTA), di-, tri-, tetra-, penta-, or hexa-N-methylol-melamine or their partially or completely etherified or esterified derivatives, for example hexamethoxymethylmelamine (HMMM).
Typically, the methylene acceptor is incorporated into the rubber compound in an amount ranging from about 1 to 25 parts by weight based on 100 parts by weight of rubber hydrocarbon (1 to 25 phr). Preferably, the methylene acceptor is incorporated into the rubber compound in an amount from about 1 to 5 phr.
Generally, the weight ratio of methylene acceptor to methylene donor is from about 1:10 to 10:1, more preferably 1:3 to 3:1.
In a preferred embodiment, a vulcanizable rubber composition is provided as described above wherein the methylene acceptor is prepared by reacting resorcinol with styrene in the presence of PTSA to produce a first reaction product, followed by further reacting the first reaction product with N-methylol caprolactam to produce a second reaction product which is the methylene acceptor.
It will be understood by those skilled in the art that the vulcanizable rubber composition of this invention may also include at least one additive comprising sulfur, carbon black, zinc oxide, silica, an anti-oxidant, a stearate, an accelerator, an oil or an adhesion promoter.
In another embodiment of this invention, a vulcanizable rubber composition is provided as described above, further comprising (d) a reinforcing material. Any reinforcing material known in the art can be used, including, but not limited to, nylon, rayon, polyester, aramid, glass, steel (brass, zinc or bronze plated) or other organic and inorganic compositions. These reinforcing materials may be in the form of filaments, fibers, cords or fabrics.
Following formation of the rubber compound, vulcanization can be carried out by methods known in the art.
It will be appreciated that the resin formed by the reaction of the methylene acceptor and methylene donor as described above promotes adhesion between the rubber and the reinforcing materials while simultaneously providing an improvement in the rubber vulcanizate properties such as hardness and dynamic stiffness, as well as improving scorch safety time and providing longer cure times when compared to the prior art. The rubber composition of the present invention further has improved adhesion properties for adhering rubber to the reinforcing materials as described above. Optionally, the reinforcing material can be pretreated or coated with adhesives.